Messages in this thread | | | Date | Wed, 11 Apr 2007 14:33:57 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] i386 tsc: remove xtime_lock'ing around cpufreq notifier |
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On Wed, 11 Apr 2007 13:54:41 -0700 Daniel Walker <dwalker@mvista.com> wrote:
> On Wed, 2007-04-11 at 13:31 -0700, Andrew Morton wrote: > > On Wed, 11 Apr 2007 09:29:04 -0700 > > Daniel Walker <dwalker@mvista.com> wrote: > > > > > The locking of the xtime_lock around the cpu notifier is unessesary now. At one > > > time the tsc was used after a frequency change for timekeeping, but the re-write > > > of timekeeping no longer uses the TSC unless the frequency is constant. > > > > > > The variables that are changed in this section of code had also once been used > > > for timekeeping, but not any longer .. > > > > > > Signed-Off-By: Daniel Walker <dwalker@mvista.com> > > > > > > --- > > > arch/i386/kernel/tsc.c | 8 +------- > > > 1 file changed, 1 insertion(+), 7 deletions(-) > > > > > > Index: linux-2.6.20/arch/i386/kernel/tsc.c > > > =================================================================== > > > --- linux-2.6.20.orig/arch/i386/kernel/tsc.c > > > +++ linux-2.6.20/arch/i386/kernel/tsc.c > > > @@ -200,13 +200,10 @@ time_cpufreq_notifier(struct notifier_bl > > > { > > > struct cpufreq_freqs *freq = data; > > > > > > - if (val != CPUFREQ_RESUMECHANGE && val != CPUFREQ_SUSPENDCHANGE) > > > - write_seqlock_irq(&xtime_lock); > > > - > > > if (!ref_freq) { > > > if (!freq->old){ > > > ref_freq = freq->new; > > > - goto end; > > > + return 0; > > > } > > > ref_freq = freq->old; > > > loops_per_jiffy_ref = cpu_data[freq->cpu].loops_per_jiffy; > > > @@ -237,9 +234,6 @@ time_cpufreq_notifier(struct notifier_bl > > > } > > > } > > > } > > > -end: > > > - if (val != CPUFREQ_RESUMECHANGE && val != CPUFREQ_SUSPENDCHANGE) > > > - write_sequnlock_irq(&xtime_lock); > > > > > > return 0; > > > } > > > > hm. > > > > I've been permadropping Andi's > > ftp://ftp.firstfloor.org/pub/ak/x86_64/quilt-current/patches/sched-clock-share > > because it causes a lockup when initscripts start ondemand on my > > single-CPU, CONFIG_SMP=n Vaio. > > > > I don't know _why_ it locks up - I traced it down to the > > write_seqlock_irq() which you have just removed. But write_seqlock() > > doesn't loop with CONFIG_SMP=n builds, so a hang there is quite mysterious. > > > > Anyway, your patch might make that hang go away. We'll see. > > > I don't know to what extent this is relevant, but it's something I've > noticed .. > > >From the patch above , > > + */ > +unsigned long long sched_clock(void) > +{ > + int cpu = get_cpu(); > + struct sc_data *sc = &per_cpu(sc_data, cpu); > + unsigned long long r; > + > + if (sc->instable) { > + /* TBD find a cheaper fallback timer than this */ > + r = ktime_to_ns(ktime_get()); > + } else { > + get_scheduled_cycles(r); > + r = ((u64)sc->ns_base) + cycles_2_ns(cpu, r - sc->last_tsc); > + } > + put_cpu(); > + return r; > +} > > Your VAIO is the "instable" case above I think .. So your using a case > that needs to be implemented still , I guess .. ktime_get() has a > peculiarity of recursively looping on the read seqlock on xtime_lock .. > > Here is the call ordering , > > ktime_get() > ktime_get_ts() -> read_seqretry(&xtime_lock, seq) > getnstimeofday() > __get_realtime_clock_ts() -> read_seqretry(&xtime_lock, seq) > > > I wonder if there is a weird case which case this to loop forever .. But > as said , it's just something I noticed so I don't know if it's > related . >
hm.
Bear in mind that printk calls sched_clock() for each line of output. (with the "time" kernel boot parameter).
If we're doing a read_seqretry() in sched_clock() then bascially any printk inside the write_seqlock() will cause a lockup.
So in fact, this explains my hang: I was debugging it with printk and I noticed that the printk before the write_seqlock() came out and the one after it did not. Presumably if I wasn't using "time", that hang wouldn't have happened.
Which means that I still don't have a clue why Andi's patch is locking up the Vaio.
It's a bad idea to make sched_clock() this complex - we've gone and degraded kernel debuggability somewhat.
We have provision for fixing this: the architecture can provide its own printk_clock(). We should do something quick-n-dirty in printk_clock() which doesn't require any locks.
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